Former king with a line on the side (7)
I believe the answer is:
lateral
'on the side' is the definition.
(I've seen this before)
'former king with a line' is the wordplay.
'former' becomes 'late' (**).
'king' becomes 'r' (r is abbreviation for rex).
'with' means one lot of letters go next to another.
'line' becomes 'l'.
'late'+'r'+'a'+'l'='LATERAL'
(Other definitions for lateral that I've seen before include "Sort of thinking" , "Shoot" , "Tall ear (anag.)" , "One type of thinking" , "belonging to the side" .)